Hey, thats a neat valve cover!!
I'm no automotive engineer, but here's how the one hose "extra suck" thing was explained to me years ago by a one legged black man named Marvin.. who also had a penchent for boons farm and young girls, so don't take anything he says all that seriously.
As the engine runs, their are ports on it that suck air.. as the carb draws air into the engine.
This can thing gets all the air sucked out of it, creating a vaccume in the can.. to a certian point.. then the constant sucking on it prevents any air from rushing back into it.. so you've got a can under the hood that's just itching to suck some air when given the chance.
Now lets say you get the RPMs up like when taking off, or putting some load on the engine.. you loose, for a moment, the natural "suck" from the carb.
When the "suck" level drops from the carb, it's no longer strong enough to operate the vac wipers at "full speed", nor is it able to trap the vaccume in the can.. so the CAN begins to suck air back in.
The suction from the can compensates for the momentary loss of suction from the carb.
The line usually goes into a port on the manifold, or in some cases, into a "Y" adapter that goes to the vac wipers.
